1. Field of the Invention
This invention relates to a high-performance magnetic tape which makes it possible to attain an enhancement of recording density in the submicron region, especially to such a magnetic tape which is usable for digital VTR or HD-VTR.
2. Description of the Prior Art
Various signals such as luminance signal, color signal, linear audio signal, FM audio signal and control signal are recorded on a magnetic tape for VTR. In the case of a household VTR, luminance signal, color signal and FM audio signal are superposed on the same track. Luminance signals are relatively short (about 1 .mu.m) in recording wavelength, so that they are recorded in the superficial portion of a magnetic layer. It is well-known that color signals are long in recording wavelength and are recorded deep into the magnetic layer. In the case of the VHS system for household VTR, FM audio signals are recorded by a special wide-gap head and then the picture signal is over-written, so that part of the recorded signal is erased and the remaining portion alone is left the signal.
Hitherto, improvements of electro-magnetic conversion efficiency have been attained by such means as enhancement of Hc, increased degree of packing of magnetic powder particles, ultra-smoothening of magnetic tape surface, adoption of oblique orientation and two-ply formation of the magnetic layer. However, improvement of picture output by enhanced Hc of magnetic tape causes a reduction of output at a low frequency corresponding to the color signal region, and the produced magnetic tape, when viewed synthetically, may prove to be an imbalanced tape. Also, since enhancement of Hc invites an increase of recording current, there arises the problem of interchangeability.
It is also notable that current magnetic recording generally depends on the system which makes use of in-plane magnetization of the recording medium. According to the recording system utilizing such in-plane magnetization, however, any attempt for enhancing recording density is attended by an increase of demagnetizing field in the recording medium, making it difficult to attain enhancement of recording density over a certain level.
For breaking through such limit of recording density, there has recently been proposed the perpendicular magnetic recording system utilizing magnetization perpendicular to a recording medium plane. This perpendicular magnetic recording system has a characteristic that the demagnetizing field in the recording medium is reduced at a high recording density, so that it can be said to be a recording system essentially suited for high-density recording.
Recording medium used for perpendicular magnetic recording includes the type having a continuous film such as a Co--Cr deposited film and the type having a coated film in which the hexagonal platelet barium ferrite particles are dispersed in a resin. Recently, for the reason of practical advantages such as lower cost and higher durability, attention is drawn to the coated film type perpendicular magnetic recording medium, but practical application of this type of recording medium to the devices of a recording and reproducing system using a ring type head is unfavorable because of occurrence of distortion of output waveform. However, in view of the fact that the magnetic powder of barium ferrite suited for high-density recording is composed of the particles which are plate-shaped and ultra-fine, with the plate thickness being of the order of 100-1,000 .ANG., there is a possibility of realization of a practical coated film type longitudinal magnetic recording medium in which orientation is made longitudinally of the medium.
On the other hand, in order to improve the recording and reproducing characteristics in the short wavelength region, many attempts have been made for minimizing surface roughness of the medium in the range that the practical performance permits, and at the same time efforts have also been actively made for elevating residual flux density of the medium.
In the field of VTR tape technology, there has been developed, and already placed on the market, a VTR tape comprising a multi-ply magnetic layer possessed of luminance, color and audio signal characteristics and capable of developing a high output in the area extending from low frequency to medium to short wavelength region.
However, in the development of the state-of-the-art coated type magnetic recording media, in the case of using magnetic powder of barium ferrite which is expected to provide a further improvement of recording medium performance, there have been noted the defects that it is impossible with the longitudinally oriented magnetic tape to obtain a sufficiently high surface smoothness as compared with the existing magnetic tapes, that the magnetic energy of the tape is far smaller than that of the iron oxide or metal particulate type tapes, and that the erasing characteristics are bad due to the influence of the perpendicular magnetization component. Thus, among the magnetic tapes using barium ferrite, there existed none which had all the characteristics that would eliminate said defects while enabling improvement of the tape performance of said conventional longitudinal recording medium. It is, therefore, impossible with the presently available recording media to obtain as high a short wavelength output as anticipated.